1. Field of the Invention
The present invention relates generally to devices which utilize magnetic write heads and more particularly to computer hard disk drives.
2. Description of the Prior Art
A computer hard disk drive stores and retrieves data by positioning a magnetic read/write head over a rotating magnetic data storage disk. The head, or heads, which are typically arranged in stacks, read from or write data to concentric data tracks defined on surface of the disks which are also typically arranged in stacks. The heads are included in structures called “sliders” onto which the read/write sensors of the magnetic head are fabricated. The slider flies above the surface of the disks on a thin cushion of air, and the surface of the slider which faces the disks is called an Air Bearing Surface (ABS).
The goal in recent years is to increase the amount of data that can be stored on each hard disk and also to decrease the time involved in reading and writing data to the hard disk. It is well known in the Recording-Head industry that when a write-head starts recording data on the media, after a dormant period, it is a relatively weak writer for approximately the first 0.5 msec of writing. After that, it begins to write much better. This so-called “first-sector overwrite” problem is caused by the gradual warming up of the writer by the writing-current that passes through its coil. When the head is “cold”, the components of the head are thermally contracted, so that the spacing between the ABS and the disk is slightly increased. Since magnetic field strength decreases with the distance, this results in a decreased field strength in writing the bits being recorded, and thus more potential errors in reading the data later. As the head warms up, the resulting temperature rise gives rise to better magnetic properties in the writing head's magnetic material, as well as a physical protrusion of the writer toward the media through thermal expansion of the materials. This reduces the spacing between the writer and the media, and thus helps the data-writing process. In some severe case, this “first-sector-overwrite” problem can impede the overall magnetic performance of a hard disk drive, giving rise to increased error rates in the first few sectors where the data-writing first occurs.
To correct this problem, it has been recent industry practice to temporarily increase the write-current above the designed nominal value in the first few sectors of writing. The increased write-current will enhance the initial weak writing by giving a larger magnetomotive force to the writer's magnetic circuit. After the writer warms up, the write-current will then be scaled back to its designed nominal value. This practice has been called “write-current latching” or “kick-latching”. Here, the word “kick” in the so-called “kick-latching” is referring to the write-current overshoots used in switching from one write-current polarity to the other. The current-overshoot is intended to enhance the transition of the writing magnetic-field.
However there are problems associated with write-current-latching or kick-latching. Although it may improve the writing process during the initial sectors, it introduces another source of signal distortions which degrades the error-rate performance of the system. In today's high-data-rate disk drives, increasing the write-current can artificially change (increase or decrease) the bit spacings between neighboring data bits. With a higher current (or larger current-overshoots) being applied to these initial sectors as it is kick-latched, pattern-dependent distortions to the bits being recorded are created, through transition-shifts. This is because larger overshoots tend to spread di-bit pulses apart inadvertently, and different degrees of overshoot strength (as used in kick-latching) would then distort the bits to different degrees by this bit-spacing spreading. This distortion by large write-current is sometimes referred to as “negative nonlinear transition shift” (or “negative NLTS”), “negative” as opposed to the conventional (or positive) NLTS, in which signal-distortions are caused by neighboring bit-transitions being pulled closer together inadvertently by demagnetization field.
In short, the previous proposal of “kick-latching” by itself may not improve the overall magnetic performance of the system (as measured by error-rate), even though it can improve the overwrite property in the first sectors.
Thus, there is a need for a “precompensation latching” procedure which compensates for the signal-distortions which are introduced by kick-latching.